Recently, a new kind of electromagnetic ground plane has been developed which is known as a high-impedance or Hi-Z surface. See D. Sievenpiper and E. Yablonovitch, “Circuit and Method for Eliminating Surface Currents on Metals” U.S. provisional patent application, Ser. No. 60/079,953, filed on Mar. 30, 1998 by UCLA and a related PCT application published as WO 99/50929 on Oct. 7, 1999. This prior art structure consists of a metal ground plane covered with an array of tiny resonant cavities. These resonant cavities alter the effective electromagnetic impedance of the surface, so that it appears to have a high impedance (>>377 ohms), instead of a low impedance (≈0 ohm) like an ordinary metal surface. Because of its high impedance, the Hi-Z structure can support a finite tangential electric field at its surface, which is not possible with a smooth metal ground plane. This textured surface is important for various applications in the field of antennas. In particular, it is useful for low-profile antennas because radiating elements can be placed directly adjacent to the Hi-Z surface (i.e. spaced less than <<0.01 wavelength therefrom) without being shorted out. This provides an advantage compared to an ordinary metal ground plane, which normally requires a separation of roughly ¼ wavelength between the ground plane and the antenna, resulting in antennas that are at least ¼ wavelength thick. In addition to providing a way to produce very thin antennas, the Hi-Z surface also suppresses surface currents, which tend to interfere with the performance of the antenna by propagating across the ground plane and radiating from edges, comers, or other discontinuities. The radiation produced by these surface currents combines with the direct radiation from the antenna, and produces ripples in the radiation pattern, as well as significant radiation into the backward direction behind the ground plane. By suppressing these surface currents, one can produce antennas with much smoother radiation patterns, and with less backward radiation. In short, the antennas are both more compact and more efficient when made with a Hi-Z surface.
The Hi-Z structure can be most easily understood by considering the effective circuit that describes the resonant cavities. In the structure shown in FIG. 1, the Hi-Z surface is constructed as a lattice of overlapping “thumbtack”-like protrusions on a flat metal ground plane 22. The protrusion consist of flat metal plates 10 connected to the ground plane by metal plated vias 13. This prior art structure shown here is built using printed circuit board techniques. The printed circuit board is not shown for ease of illustration, but the flat metal plates 10 would appear on the printed circuit board's top surface while the ground plan 22 is disposed on its bottom surface. The capacitance of the structure is determined by the proximity and overlap area of the metal plates 10. The inductance is controlled by the area of the current loop that connects adjacent plates, which is primarily determined by the thickness of the structure. The resonance frequency of the surface is then given by
  ω  =            1              LC              .  Near the resonance frequency, the surface has high impedance, and can suppress the propagation of surface currents. The bandwidth of the surface, or the frequency band where the impedance is greater than 377 ohms, is given by
  BW  =                              L          /          C                                                  μ            o                    /                      ɛ            o                                .  This roughly determines the bandwidth of antennas that can be built on these surfaces.
Typically, in the prior art, Hi-Z surfaces are produced by printed circuit board techniques. In order to achieve a low resonant frequency (<10 GHz or so) in a thin structure (a few mm thick), a large amount of built-in capacitance is required. This is accomplished using a multi-layer structure, in which the capacitors are of a parallel-plate geometry. The vias 12 are made by drilling through both boards, and then plating the inside of the holes with metal 13. The steps taken in fabrication are shown in FIGS. 2(a)–2(f). First, two printed circuit boards, one relatively thick and one relatively thin form the starting materials (see FIG. 2(a)). The inner layers are patterned (see FIG. 2(b)), and the boards are bonded together (see FIG. 2(c)). Then holes 12 are drilled through the structure to define the positions of the vias (see FIG. 2(d)). These are then plated with metal 13 (see FIG. 2(e)). Finally, the outer layers are patterned (see FIG. 2(f)). The most time-consuming and expensive task is drilling the vias 12. A fast computer-controlled drill can drill on the order of one hole per second. Typical lattice periods for these structures are on the order of ¼ inch, which means that the total drilling time can approach one hour per square foot.
What is needed is a method of producing a similar structure by faster and more economic techniques, in which the holes do not need to be drilled individually, but instead can be produced en masse by some other technique. This invention provides techniques for producing such a structure by molding, as well as new geometries that are amenable to such manufacturing techniques. The resulting structure is less expensive and less time-consuming to fabricate. Furthermore, it has the additional benefit that certain embodiments thereof can be tuned after fabrication to adjust for variations in the manufacturing process. This feature also allows a single mold to be used to build structures with slightly different resonant frequencies.